The use of solar cells for powering instrumentation aboard earth-orbiting spacecraft is well known and an established practice and has been extended to power long-range spacecraft even to the outer planets. However, the cost of solar cells per watt of sustained output is very great and completely noncompetitive in the earthbound environment, at least to date. The minimum cost per watt of sustained power under current technology is of the order of $15 per watt, and while improved techniques and manufacturing are being exploited, with the ongoing inflation the cost per watt is, if anything, likely to rise further. One can suppose that there will be breakthroughs in the ultimate techniques of reduced cost in production of silicon collectors, including improvements in the quantum efficiency of silicon-coated substrates, but until these are established, it is worthwhile to explore what might be done to concentrate sunlight in advance onto silicon collectors of reduced area.
In the existing art, Fresnel plastic leans sheets are being successfully used and indeed produce probably the best gain to date in overall economy, as compared, say, to reflective concentrators in the form of cylindrical troughs of generally parabolic cross section. Indeed, the use of such parabolic cylinders comes from the older art introduced toward the turn of the last century by Dr. Charles G. Abbott, director for many decades of the Smithsonian Astrophysical Observatory. Abbott made use of a parabolic cylindrical trough for focusing sunlight onto a central straight pipe lying along the focal line. The pipe was part of a plumbing circuit that conveyed hot oil into a heat exchange arrangement. While a parabolic cylinder is remarkably simple for its purpose and does indeed produce an adequately focused image of the sun onto a focal line, the cylinder nevertheless must be mounted and driven in either one or two axes to track the sun throughout the day and seasons. If the focal line of the cylinder is mounted parallel to the axis of the earth, then one-dimensional tracking is sufficient for following the sun from dawn to dusk and throughout the year.
An entire array of cylindrical parabolic troughs can be set up in a flat area, such as a field or roof, or even on a sloping roof, and driven to track either in the above-described single mode, or just as easily with modern electronic controls in two coordinates--the only additional requirement being that the strip image of the sun lie along the focal line. There are certain practical difficulties, however, having to do with rain, wind, ice and snow, and with dust and corrosion of the reflectors. Of these, perhaps wind is the worst offender in that countermeasures lead to high cost and elaborate arrays. For power purposes these costs are barely competitive but numerous arrays have been and are being built here and abroad.
A competing means has been devised to minimize the extensive plumbing requirements of an array of troughs. Here, thousands of flat or perhaps slightly curved mirrors are arrayed to focus sunlight onto a single collector, which necessarily is exposed to intense heat. The energy focused onto the collector is removed at a sufficient rate so that the collector is maintained below a damaging temperature. This kind of array is under development for power purposes and indeed has been long used by the French in an impressive installation in southern France in the Pyrenees. This kind of array is almost too intense for use with solar cells and is better suited to indirect means for power generation such as the use of super-heated steam to drive turbines. There also are the devices that simply make use of the so-called greenhouse effect to cause the temperature to rise within a tube or set of concentric tubes. The tubes contain some fluid for heat transport to a central station. There has also been proposed to make use of the same intensely active molten mixture of sodium and magnesium that is used in heat transport aboard nuclear submarines. Such tubes are usually mounted along the focal lines of reflective or Fresnel concentrators and are not principally used for photovoltaic conversion.
Returning therefore to to use of Fresnel plastic lens sheets, it is to be noted that this method is cost effective and already within the state of the art. Nevertheless, tracking in one or two degrees of freedom is still involved, and may lead to costly mounting problems to counteract the distructive effects of wind, snow and ice. Arrays of Fresnel concentrators can be mounted in polar form to track the sun throughout the day onto silicon converters mounted along focal lines, or can be mounted in gimbals to track the sun in two degrees of freedom throughout the day. One may note that only one precision tracker to follow the sun is needed, if all the other elements of the array, however large, are slaved to the precision tracker, either through gearing, belt drives or preferably by electronic controls. If the Fresnel sheets are large, they must be adequately mounted and protected from wind damage. If they are small, one is faced with having numerous mechanical or electrical drives in one or two coordinates, which of themselves must be sturdy enough to withstand all sources of degradation.
As will be seen in the detailed description which follows, the type of solar tracker and concentrator being considered here is probably most closely related to the Fresnel system but is in the form of flat panels that can be flat mounted and arranged in large arrays, whether on a field, or on a flat or sloping roof, or used as windows or skylights, or even as canopies over fields. While the threat of wind damage is always present, well anchored, flat-streamlined panels, and sets of panels provide no individual edges that will allow the wind to work its will ultimately on the array. Indeed, protective fencing around a field of flat panels can be used to calm ambient wind conditions to harmless breezes.
One object of this invention, therefore, is to provide for solar tracking in the form of panels which move only slightly in their own planes for tracking the sun throughout the day.
Another object of this invention is to miniaturize the lateral motion required of a solar tracking panel to allow almost imperceptible rates and amplitudes so as to confine the range of movement of individual panes or lights of the panels so that they may be mounted within simple framing, as for ordinary window frames, without having the appearance or reappearance of exposed edges open to view or that would collect dirt, snow and ice and interact with the wind.
Other objects of the invention will in part be obvious and will in part appear hereinafter. The invention, accordingly comprises the apparatus possessing the construction, combination of elements, and arrangement of parts exemplified in the following detailed disclosure.